1. Field of the Invention
The present invention relates to a nuclear magnetic resonance (NMR) spectrometer and, more particularly, to an NMR probe which is placed within a static magnetic field while holding a sample therein and which permits irradiation of the sample by RF pulses and detection of the resulting NMR signal.
2. Description of the Related Art
An NMR probe is a device used for detection of an NMR signal. In use, it is inserted in a high field magnet, such as a superconducting magnet. Since the static magnetic field applied to the NMR probe is required to have a high degree of homogeneity, the NMR probe is entirely made of a material having a low magnetic susceptibility to prevent distortion of the static magnetic field.
FIG. 1 shows main portions of the prior art NMR probe. A support 1 is located at the top of the NMR probe and acts to hold a coil bobbin 3 on which a detector coil 2 is mounted. The coil 2 is used to detect an NMR signal. An extraction line a that is a signal line extends from the detector coil 2. The support 1 is provided with a hole b to pass the extraction line a. This line is electrically connected with an electric circuit portion c mounted at the bottom of the support 1.
A cold gas and a hot gas for varying the temperature are admitted into the space surrounded by the support 1 and the double tube coil bobbin 3 via a glass tube 4 to investigate a sample tube d at different temperatures. Since it is necessary to vent the gases without leakage, the hole b for passing the extraction line a laid in the support 1 is normally filled with an adhesive. Thus, the portion surrounding the sample tube d is closed.
In this structure, the extraction line a passes through the support 1. In spite of this, the support 1 is required not to produce any background signal that cannot be distinguished from the NMR signal arising from the sample. Furthermore, the support is required to have sufficient heat resistance such that it does not deform even if it is exposed to cold and hot gases during measurements under variable-temperature conditions. The magnetic field in the vicinities of the detector coils 2 must have a very high degree of uniformity. Where a magnetic field gradient pulse is applied during a measurement, for example, if the support 1 is made of a metallic material, an eddy current will be produced, distorting the uniformity of the static magnetic field. This will deteriorate the resolution of the NMR signal, thus hindering the measurement. Therefore, it is necessary that the support 1 be made of a nonmagnetic material producing no eddy current. If a material having a large dielectric loss is used, the Q factor of the NMR probe decreases, lowering the detection sensitivity. Consequently, a material having a small dielectric loss must be employed.
For these reasons, limitations are imposed on the usable materials today. Materials satisfying all the requirements described above are almost unavailable. Especially, with respect to the problems of NMR background, almost all materials contain .sup.1 H nuclei (proton) to be investigated by NMR and so NMR background signals are detected from almost every material. Hence, the usable material is selected from a very narrow choice of materials.
For example, where the support 1 is made of Teflon, it exhibits excellent heat resistance during measurements under variable-temperature conditions and excellent machinability when the material is machined. However, the material produces a considerable amount of background signal due to .sup.1 H nuclei (proton) that are main nuclei observed by NMR. Where Diflon is used, a smaller amount of background signal is produced from .sup.1 H nuclei than where Teflon is used. However, the heat resistance is inferior and, therefore, the material cannot be elevated to high temperatures. Whether Teflon or Diflon is used, the hole b in the support 1 for the extraction line a is filled with an adhesive to prevent leakage of temperature-varying gases. This induces NMR background signals.